Browsing by Author "Cheng, Sheng-Lung"

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An Enhanced Zero-forcing Equalizer for Combinerless LINC-OFDM Systems
(2014)
Cheng, Sheng-Lung
;
Wu, Wen-Rong
;
Hsu, Ying-Pei
;
電信工程研究所
;
Institute of Communications Engineering
The energy efficiency of OFDM systems is known to be low due to the high peak-to-average power ratio problem. The linear-amplification-with-nonlinear-component (LINC) technique can solve the problem by decomposing the input signal into two constant-envelop component signals and applying high-efficient nonlinear amplifiers. However, the power combiner, a key component used to combine the amplified signals, is difficult to implement. Combinerless LINC systems employ two transmit antennas such that two component signals can be naturally combined at the receiver. Unfortunately, the performance of combinerless LINC-OFDM systems is seriously degraded if difference exists, even small, between the two channels that the two component signals propagate. In this paper, we propose an enhanced zero-forcing equalizer to solve this problem. The main idea is to reduce a self-interference induced by the channel difference. A closed-form expression for the optimum parameter of the proposed equalizer is also derived. Simulations show that the proposed equalizer can effectively enhance the performance of combinerless LINC-OFDM systems.
Digital Predistortion for Concurrent Dual-band Transmitter Using a 2D LUT Based Method
(2015)
Cheng, Sheng-Lung
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Wu, Wen-Rang
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Peng, Chun-Hsien
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Hsu, Chen-Jui
;
Liang, Paul
The digital predistortion (DPD) technique has been utilized in radio-frequency power amplifier linearization for its low cost and good performance. However, the conventional singleband OPO design is not applicable to the concurrent dual-band transmitters since the cross modulations are induced. The twodimensional (20) polynomial based OPO has been proposed as a remedy. Despite its good performance, the main drawbacks are the high computational complexity for the identification of the DPD coefficients and the high implementation complexity. It is known that the look-up-table (LUT) based method, which has been applied in conventional single-band transmitters, can effectively overcome the problems. In this paper, we consider a 20 LUT based DPD for the concurrent dual-band transmitters. First, we formulate a cost function from which the elements of the tables can be obtained through the optimization. Then, we propose an adaptive algorithm using the gradient-descent (GO) method to conduct the search. Moreover, we propose a practical method to implement the GO method which can significantly reduce the computational and implementation complexities. Finally, some measurement results are presented to demonstrate that the proposed algorithm has the satisfactory performance.
Digital predistortion for concurrent dual-band transmitter using a 2D-LUT based Hammerstein architecture
(2017-01)
Cheng, Sheng-Lung
;
Wu, Wen-Rong
;
Peng, Chun-Hsien
;
Liu, Jen-Yang
;
Hsu, Chen-Jui
Digital predistortion has been proved to be an effective remedy for the nonlinear distortion in radio-frequency power amplifier. Unfortunately, conventional digital predistorters (DPDs) developed for single-band transmitters can not be directly applied to concurrent dual-band. To solve the problem, DPDs with two dimensional memory polynomials were then proposed. Although they can provide good performance, its computational complexity is high for the estimation of polynomial coefficients. Look-up-table (LUT) based DPDs, developed for single-band transmitters, can effectively solve the problem. However, there are limited studies about LUT based DPDs for concurrent dual-band transmitters. In this paper, we propose a two-dimensional LUT based Hammerstein DPD to solve the problem. Analyzing the signals on both bands, we propose a new adaptive algorithm with the gradient-descent and recursive-least-squares (RLS) methods to train the DPD iteratively. With the proposed method, the computational complexity in the identification of the DPDs can be significantly reduced. Finally, simulations and experiments are conducted to demonstrate that the performance of the proposed DPD is comparable to that of the polynomial based DPD.
Maximum likelihood detection for coded combinerless LINC-OFDM systems
(2016-06-29)
Wu, Wen-Rong
;
Cheng, Sheng-Lung
;
Hsu, Ying-Pei
;
電信工程研究所
;
Institute of Communications Engineering
Owning to the high peak-to-average power ratio problem, the power efficiency of orthogonal-frequency-division-multiplexing ( OFDM) systems is usually low. It deteriorates in millimeter wave systems in which the design of an efficient linear power amplifier is much more challenging. The linear-amplification-with-nonlinear-component ( LINC) technique can serve as a remedy, decomposing the input signal into two constant-envelop component signals followed by high-efficient nonlinear amplifiers. However, the power combiner, a key component used to combine the amplified signals, is difficult to implement. Combinerless LINC systems employ two transmit antennas such that two component signals can be naturally combined at the receiver. Unfortunately, the performance of combinerless LINC-OFDM systems is seriously degraded if difference, even small, exists between the two channels. The maximum likelihood ( ML) receiver can effectively solve the problem; however, its computational complexity is prohibitedly high. We propose a coded combinerless LINC-OFDM system, including a convolutional encoder and a list Viterbi algorithm ( LVA) decoder, to solve the problem. The LVA can provide a small number of candidates for the ML detector, dramatically reducing the required computational complexity. We also utilize an enhanced zero-forcing equalizer such that the soft-demapping operation can be effectively conducted. Finally, we propose a simple iterative interference cancellation scheme to further enhance the performance. Simulations show that the proposed combinerless LINC-OFDM system can outperform the conventional OFDM while the consumed power is much lower.